Today I would like to introduce another major system considering items! This is a double conception, since Crafting and Inventory are separate in some ways, but closely related to each other.
There'll be a new, small button besides your "Shop" interface where you buy towers and items in the game that reads "Crafting". Once you click this button, you'll go to the Crafting Table interface and the game will pause. Once in the Crafting Table, you can click the Recipes button, click one of your recipes to select it (recipes can be gotten by doing things such as use x item y times, pop x bloons of y type, reach Rank n, complete mission z, etc...) Then you'll drag the necessary ingredients from the Inventory to each point of the Star of David, and adjust their quantities. Each point of the star is for one type of ingredients only, so the craftable items with the most types of ingredients should only have 6 types of ingredients. When you're done, click the circle in the center and you crafted item will appear there, depending on the quantity of ingredients (For example, if the item z needs n item x and n' item y to craft, then if you use 10n item 10x and 10n' item y, 10 item z will appear.) The crafted items will then be added to your Inventory. Then you can click Return to unpause the game and exit the Crafting Table, or you can craft another item. All Crafted Items will stay in your Inventory forever, which means they won't disappear once you complete a game but still hasn't used it!
Success Rate and Catalyst
Not every items will have a 100% Success Rate, which means it'll always be created if the necessary ingredients are crafted. Some items have much lower Success Rate, and if the success rate is, for example, 50%, then that item will have a 50% chance to be successfully created when crafting the required ingredients. Otherwise, if it's a failure, all of the ingredients will be destroyed, but you won't get anything.
However, some recipes can actually create TWO or more items! For example, there's a recipe A that has a 100% success rate to create an item. But, there's a 80% chance that A will create item X, and there'll be a 20% chance that A won't create X, but won't fail either, and will instead create item Y (better than X). This will allow a single recipe to create multiple closely related items.
Catalyst Ingredients are items that don't act as an active ingredient for that recipe, but rather, can be crafted together with the normal ingredients to improve the Success Rate. You don't need the Catalysts to craft an item, but Catalysts will help you have more chance to get what you need. How much will the Success Rate be improved is based on the crafted item's Class. Note that a Catalyst for this recipe can be an active ingredient for another recipe.
The Inventory button is just above the Crafting button on the main interface. By clicking it you can view your Inventory which has 20 slots, each slot for 1 type of items. Loot Items and Crafted Items will automatically go to the inventory. And if you want to buy an item as ingredient for Crafting and don't use it instantly, you can hold shift while buying that item to put it in your Inventory instead of using it. You can also drag an item from the Inventory to the track to use it, of course (if you can't, then Crafting would be completely useless, right?)! Simple, right?